MIPS Function Continued

Character and String Operations 4/11/2017 Character and String Operations Characters are encoded as 0’s and 1’s using ASCII most commonly American Standard Code for Information Interchange Each character is represented using 8 bits (or a byte) MIPS provides instructions to move bytes Load byte (lb) loads a byte to the rightmost 8 bits of a register Store byte (sb) write the rightmost 8 bits of a register to memory week04-3.ppt 4/11/2017 CDA3100 week04-3.ppt

SPIM syscalls li $v0,1 # print an integer in $a0 li $a0,100 syscall li $v0,5 # read an integer into $v0 li $v0,4 # print an ASCIIZ string at $a0 la $a0,msg_hello li $v0,10 #exit

String Copy Procedure week04-3.ppt 4/11/2017 4/11/2017 CDA3100 week04-3.ppt

.asciiz "Hello! This is CDA3100!\n" msg_empty: .space 400 .text .data msg_hello: .asciiz "Hello! This is CDA3100!\n" msg_empty: .space 400 .text .globl main main: li $v0,4 la $a0,msg_hello syscall la $a0,msg_empty la $a0,msg_empty #dst la $a1,msg_hello #src jal strcpy li $v0,10 #exit strcpy: ori $t0, $a0, 0 ori $t1, $a1, 0 strcpyloop: lb $t3, 0($t1) sb $t3, 0($t0) beq $t3, $0, strcpydone addi $t0, $t0, 1 addi $t1, $t1, 1 j strcpyloop strcpydone: jr $ra

Stack Key things to keep in mind: Stack is a software concept – last in first out, that’s it. In MIPS, you implement the stack by yourself by keeping $sp always pointing to the top element on the stack Stack can be used in functions to save register values, and is the standard approach to save register values. But You can also use stack for other purposes This is not the only way to save register values.

msg: .asciiz "hello world" endl: .asciiz "\n" .text .globl main main: .data msg: .asciiz "hello world" endl: .asciiz "\n" .text .globl main main: addi $sp,$sp,-1 sb $0,($sp) la $t1, msg L0: lb $t0,($t1) beq $t0,$0, L1 sub $sp,$sp,1 sb $t0,($sp) add $t1,1 j L0 L1: la $t1,msg L2: lb $t0,($sp) add $sp,$sp,1 sb $t0,($t1) beq $t0, $0, L3 add $t1,1 j L2 L3: la $a0,msg li $v0,4 syscall la $a0,endl li $v0,10 #exit

Implementing a Recursive Function Suppose we want to implement this in MIPS: It is a recursive function – a function that calls itself. It will keep on calling itself, with different parameters, until a terminating condition is met.

The Recursive Function What happens if we call fact(4)? First time call fact, compare 4 with 1, no less than 1, call fact again – fact(3). Second time call fact, compare 3 with 1, no less than 1, call fact again – fact(2). Third time call fact, compare 2 with 1, no less than 1, call fact again – fact(1). Fourth time call fact, compare 1 with 1, no less than 1, call fact again – fact(0). Fifth time call fact, compare 1 with 1, less than 1, return 1. Return to the time when fact(0) was called (when calling fact(1)). Multiply 1 with 1, return 1. Return to the time when fact(1) was called (when calling fact(2)). Multiply 2 with 1, return 2. Return to the time when fact(2) was called (when calling fact(3)). Multiply 3 with 2, return 6. Return to the time when fact(3) was called (when calling fact(4)). Multiply 4 with 6, return 24.

The Recursive Function In MIPS, we say calling a function as going to the function. So we go to the function over and over again, until the terminating condition is met. Here, the function is called “fact,” so we will have a line of code inside the fact function: jal fact

The Recursive Function The parameter should be passed in $a0. In the C function, every time we call fact, we call with n-1. So, in the MIPS function, before we do “jal fact”, we should have “addi $a0, $a0,-1.” addi $a0, $a0, -1 jal fact

The Recursive Function After calling fact, we multiply the return result with n, so addi $a0, $a0, -1 jal fact mul $v0, $v0, $a0

The Recursive Function After multiplying, we return, so addi $a0, $a0, -1 jal fact mul $v0, $v0, $a0 jr $ra

The Recursive Function So, one if else branch is done. The other branch is slti $t0, $a0, 1 beq $t0, $0, L1 ori $v0, $0, 1 jr $ra where L1 is where we should go to if n >= 1.

The Recursive Function fact: slti $t0, $a0, 1 beq $t0, $zero, L1 ori $v0, $0, 1 jr $ra L1: addi $a0, $a0, -1 jal fact mul $v0, $v0, $a0 Any problems?

The Recursive Function fact: addi $sp, $sp, -4 sw $ra, 0($sp) slti $t0, $a0, 1 beq $t0, $zero, L1 ori $v0, $0, 1 lw $ra, 0($sp) addi $sp, $sp, 4 jr $ra L1: addi $a0, $a0, -1 jal fact mul $v0, $v0, $a0 The problem is that the function will call itself, as we have expected, but it will not return correctly! You need to save $ra, because you made another function call inside the function. You should always do so. Is this enough?

The Recursive Function So now you can return to the main function, but the return result is 0, why? Because you did not return to the correct time. Time, here, means what was the parameter you called fact with. So, should also save $a0!

The Recursive Function 4/11/2017 The Recursive Function week04-3.ppt 4/11/2017 CDA3100 week04-3.ppt

The Stack During Recursion 4/11/2017 The Stack During Recursion week04-3.ppt 4/11/2017 CDA3100 week04-3.ppt

.data .text .globl main main: li $a0, 4 jal fact done: li $v0,10 syscall fact: addi $sp, $sp, -8 sw $ra, 4($sp) sw $a0, 0($sp) slti $t0, $a0, 1 beq $t0, $zero, L1 ori $v0, $0, 1 addi $sp, $sp, 8 jr $ra L1: addi $a0, $a0, -1 lw $ra, 4($sp) lw $a0, 0($sp) mul $v0, $v0, $a0

Two other MIPS pointers $fp: When you call a C function, the function may declare an array of size 100 like int A[100]. It is on the stack. You would want to access it, but the stack pointer may keep changing, so you need a fixed reference. $fp is the “frame pointer,” which should always point to the first word that is used by this function. $gp: the “global pointer.” A reference to access the static data.